Perfect fifth

From AudioLexic

The perfect fifth or diapente is a musical interval which is responsible for the most consonant, or stable, harmony outside of the unison and octave. It is a valuable interval in chord structure, song development, and western tuning systems. The prefix perfect identifies it as belonging to the group of perfect intervals (Perfect fourth, Perfect octave) so called because of their extremely simple pitch relationships resulting in a high degree of consonance.

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The perfect fifth is a basic element in the construction of major and minor triads, and because these chords occur frequently in much music, the perfect fifth interval occurs just as often. However, due to its high level of consonance, the perfect fifth contributes very little to the overall harmonic effect of any chords containing it (except power chords). Because of this, in any situation that necessitates the omission of notes from a chord (such as for practical reasons of fingering) the note forming the perfect fifth above the chord's root can often be safely omitted, its absence being barely, if at all, noticeable.

A bare fifth, open fifth or empty fifth is a chord containing only a perfect fifth with no third. The closing chord of Mozart's Requiem is an example of a piece ending on an empty fifth, though these "chords" are common in Christian Sacred Harp singing and throughout rock music, especially hard rock, metal, and punk music, where overdriven or distorted guitar can make thirds sound muddy, and fast chord-based passages are made easier to play by combining the four most common guitar hand shapes into one. Rock musicians refer to them as power chords and often include octave doubling (i.e. their bass note is doubled one octave higher, e.g. F3-C4-F4).

A perfect fifth in just intonation, a just fifth, corresponds to a pitch ratio of 3:2, while in 12-tone equal temperament, a perfect fifth is equal to seven semitones, or 700 cents, which is equivalent to a pitch ratio of 1:27/12 (approximately 1.4983), about two cents smaller than the just fifth. Because the pitch ratio is a ratio of small numbers, the perfect fifth is harmonically significant.